1. Technical Field
The present invention relates generally to semiconductor technology and more specifically to preventing bitline short circuits in MirroBit(copyright) Flash memory.
2. Background Art
Various types of memories have been developed in the past as electronic memory media for composits and similar systems. Such memories include electrically erasable programmable read only memory (EEPROM) and electrically programmable read only memory (EPROM). Each type of memory had advantages and disadvantages. EEPROM can be easily erased without extra exterior equipment but with reduced data storage density, lower speed, and higher cost. EPROM, in contrast, is less expensive and has greater density but lack erasability.
A newer type of memory called xe2x80x9cFlashxe2x80x9d EEPROM, or Flash memory, has become extremely popular because it combines the advantages of the high density and low cost of EPROM with the electrical ersability of EEPROM. Flash memory can be rewritten and can hold its contents without power. It is used in many portable electronic products, such as cell phone, portable computers, voice recorders, etc. as well as in many larger electronic systems, such as cars, planes, industrial control systems, etc.
In Flash memory, bits of information are programmed individually as in the older types of memory, such as dynamic random access memory (DRAM) and static random access memory (SRAM) memory chips. However, in DRAMs and SRAMs where individual bits can be erased one at a time, Flash memory must currently be erased in fixed multi-bit blocks or sectors.
Conventionally, Flash memory is constructed of many Flash memory cells where a single bit is stored in each memory cell and the cells are programmed by hot electron injection and erased by Fowler-Nordheim tunneling. However, increased market demand has driven the development of Flash memory cells to increase both the speed and the density. Newer Flash memory cells have been developed that allow more than a single bit to be stored in each cell.
One memory cell structure involves the storage of more than one level of charge to be stored in a memory cell with each level representative of a bit. This structure is referred to as a multi-level storage (MLS) architecture. Unfortunately, this structure inherently requires a great deal of precision in both programming and reading the differences in the levels to be able to distinguish the bits. If a memory cell using the MLS architecture is overcharged, even by a small amount, the only way to correct the bit error would be to erase the memory cell and totally reprogram the memory cell. The need in the MLS architecture to precisely control the amount of charge in a memory cell while programming also makes the technology slower and the data less reliable. It also takes longer to access or xe2x80x9creadxe2x80x9d precise amounts of charge. Thus, both speed and reliability are sacrificed in order to improve memory cell density.
An even newer technology allowing multiple bits to be stored in a single cell is known as xe2x80x9cMirrorBit(copyright)xe2x80x9d Flash memory has been developed. In this technology, a memory cell is essentially split into two identical (mirrored) parts, each of which is formulated for storing one of two independent bits. Each MirrorBit Flash memory cell, like a traditional Flash cell, has a gate with a source and a drain. However, unlike a traditional Flash cell in which the source is always connected to an electrical source and the drain is always connected to an electrical drain, each MirrorBit Flash memory cell can have the connections of the source and drain reversed during operation to permit the storing of two bits.
The MirrorBit Flash memory cell has a semiconductor substrate with implanted conductive bitlines. A multilayer storage layer, referred to as a xe2x80x9ccharge-trapping dielectric layerxe2x80x9d, is formed over the semiconductor substrate. The charge-trapping dielectric layer can generally be composed of three separate layers: a first insulating layer, a charge-trapping layer, and a second insulating layer. Wordlines are formed over the charge-trapping dielectric layer perpendicular to the bitlines. Programming circuitry controls two bits per cell by applying a signal to the wordline, which acts as a control gate, and changing bitline connections such that one bit is stored by source and drain being connected in one arrangement and a complementary bit is stored by the source and drain being interchanged in another arrangement.
Programming of the cell is accomplished in one direction and reading is accomplished in a direction opposite that in which it is programmed.
A major problem with the MirrorBit architecture has been discovered as the memory cells are scaled down in size or shrunk and short circuits have started to occur between the bitlines leading to corrupted data and inoperable memory cells. This appears to be a result of switching from a xe2x80x9csilicidexe2x80x9d process to a xe2x80x9csalicidexe2x80x9d process.
Traditionally, the silicide process has been used in the manufacture of the contact areas for the contacts to the polysilicon wordlines. The silicide process involves the co-deposition of a metal and silicon onto the polysilicon or the silicon substrate. The most commonly formed silicide is tungsten silicide (WSi). However, the electrical resistance of the tungsten silicide increases faster as the contact areas get smaller because to the grains of the material are large enough to cause resistance to electron flow. The relatively higher electrical resistance in smaller devices results in higher power requirements, and subsequent heat generation with reduced life expectancy, for the memory cell.
The higher electrical resistance has prompted a switch to different metal silicides, which have smaller grain sizes and so exhibit lower electrical resistance with smaller contact areas. The newest suicides are cobalt silicide (CoSi), titanium silicide (TiSi) and nickel silicide (NiSi). These silicides are deposited uniformly over the semiconductor wafer and the metal reacts with or xe2x80x9cgrowsxe2x80x9d on silicon. The process has the advantage that it will only form silicides on the polysilicon of the wordline or the silicon of the semiconductor substrate so the silicides are generally self-aligned without the need for masking. Thus, the process is called a xe2x80x9csalicidexe2x80x9d process for self-aligned silicide process.
Unfortunately, the salicide process advantage also creates a problem. It has been discovered that, since the metal is deposited uniformly over the semiconductor wafer the metal often leaks during deposition down to the silicon substrate and forms silicides where they are not desired. In particular, silicide between the bitlines causes short circuits and results in the corrupted data and inoperable memory cells.
A solution to this problem has been long sought but has long eluded those skilled in the art.
The present invention provides a manufacturing method for semiconductor devices, which includes providing a semiconductor substrate and depositing a charge-trapping dielectric layer. First and second bitlines are implanted and a wordline layer is deposited. A hard mask layer is deposited over the wordline layer. The hard mask is of a material formulated for removal without damaging the charge-trapping dielectric layer. A photoresist is deposited over the wordline layer and used to form a hard mask. The photoresist is removed. The wordline layer is processed using the hard mask to form a wordline and the hard mask is removed. A salicide can be grown without short-circuiting the first and second bitlines.